Sen. Barack Obama offers himself as a post-partisan uniter who will solve the country's problems by reaching across the aisle and beyond the framework of liberal and conservative labels he rejects as useless and outdated.

But as Obama heads into the final presidential primaries, Sen. John McCain and other Republicans have already started to brand him a standard-order left-winger, "a down-the-line liberal," as McCain strategist Charles R. Black Jr. put it, in a long line of Democratic White House hopefuls.

Sen. Hillary Rodham Clinton's campaign has also started slapping the L-word on Obama, warning that his appeal among moderate voters will diminish as they become more aware of liberal positions he took in the past, such as calling for single-payer health care and an end to the U.S. embargo against Cuba. "The evidence is that the more [voters] have been learning about him, the more his coalition has been shrinking," Clinton strategist Mark Penn said.

The double-barreled attack has presented Democratic voters with some persistent questions about Obama: Just how liberal is he? And even if he truly is a new kind of candidate, can he avoid being pigeonholed with an old label under sustained assault?

Despite being rated the most liberal senator in 2007 by the National Journal, Obama has sought to confound easy categorization. While his record and platform mostly adhere to a left-leaning Democratic model, he has cast them as a common-sense response to the Bush administration. His ability to appeal to independents and even Republicans has been one of his main attractions for Democrats eager to retake the White House, and a cause for concern among some GOP leaders.

At the same time, the criticism from the McCain and Clinton operations draws a quick rebuttal from Obama's campaign. His strategists recognize that Democratic voters and the superdelegates who may end up deciding the hotly contested nomination are concerned about the electability of a candidate tagged with the "liberal" label that has fatally wounded nominees such as John F. Kerry, Michael S. Dukakis and Walter F. Mondale.

"While there's no doubt that Obama comes from a progressive bent, he's got a very rich and thoughtful approach, and that's the reason why you have both Democrats and Republicans who've worked with him who say positive things about him," said David Axelrod, Obama's chief strategist. He chided the Clinton camp for warning about Obama's liberalism, saying she would be subject to the same critique.

"The Republican Party would very much like to run against Hillary Clinton . . . and would have no trouble taking individual votes to create the kind of caricature they want to create," he said. "It's laughable to suggest that somehow she would be impervious to that. She wouldn't be. They would have a field day."

Obama's elusiveness until now has been a source of frustration for Clinton. While her campaign now argues that Obama is too liberal, Clinton has mixed this message by attacking from the left on several issues, such as suggesting that he is weak on abortion rights, too fond of Ronald Reagan and too timid on health-care reform.

"The frustration that the Clinton campaign has felt . . . comes very much from trying to attack him from the left and right along the traditional spectrum," said Robert Reich, who served as labor secretary in Bill Clinton's administration. "But he's playing an entirely different game, and they don't know how to play that game."

In most major areas, Obama has taken positions that would seem to conform to the Republican stereotype of a liberal. Like Clinton, he favors expanding the government's role in delivering health care, and would pay for that by ending President Bush's tax cuts for the rich. He would go a step further than Clinton by lifting the limit on income taxed for Social Security, now $100,000, to set that program on firm footing.

He strongly supports abortion rights and spoke out against a Supreme Court ruling last year that upheld a ban on the procedure that some call "partial-birth" abortion. He favors allowing illegal immigrants to get driver's licenses (after some hesitation, Clinton came out against that). He is outspoken on civil rights, and he has opposed Bush's judicial picks, staying out of a bipartisan effort to approve some nominees. While he supports the death penalty for the most "heinous" crimes, as a Senate candidate in 2004 he expressed support for strict gun control, decriminalizing marijuana and ending federal mandatory minimum prison sentences, issues he now rarely raises on the trail.